1. Field of the Invention
The disclosure herein relates generally to printing technology, and more specifically to an apparatus and method for selectively ejecting ink droplets in response to pulses of electromagnetic energy.
2. Description of the Related Art
Ink-on-demand printing systems provide the ability to print on a variety of media under computer control. In commercially available ink-on-demand printing systems, two primary approaches are used. In one approach, thermal heaters are used to eject ink droplets from an orifice by the explosive formation of a vapor bubble within the ink supply. Typically, the heating of the ink is performed by resistive heating, e.g., by applying an electrical pulse to a resistor in contact with the ink supply. Such systems are described more fully in U.S. Pat. No. 4,490,728 issued to Vaught et al. and in U.S. Pat. No. 4,723,129 issued to Endo et al., both of which are incorporated in their entirety by reference herein. In addition, U.S. Pat. No. 4,351,617 issued to Landa describes a ballistic impact printer which paved the way for such thermal ink-jet systems.
An alternative approach utilizes mechanical displacements of the ink by employing piezoelectric crystals to propel ink from an orifice of a tube of narrow cross-section. Such systems are described more fully in various U.S. patents assigned to Epson Corp., including U.S. Pat. No. 6,402,304 issued to Qiu et al. and U.S. Pat. No. 5,255,016 issued to Usui et al., both of which are incorporated in their entirety by reference.
Despite the fact that both of these approaches have been known for many years, the technology of ink-on-demand ink-jet printing has yet to resolve the fundamental problems associated with these approaches. For example, for thermal systems, a 0.1 millimeter bubble expands in about 1 microsecond, collapses in about 10 microseconds, and the meniscus relaxes in about 100 microseconds after 4 or 5 oscillations (the meniscus serves as a pump to draw new ink into the nozzle). Thus, the bubble collapse time and the meniscus limit the rate of droplet ejection to approximately 4 kHz. In contrast, piezoelectric resonators, which are not sensitive to the nozzle meniscus, can operate at about 75 kHz (limited by the volumetric speed, that is, the change of volume per unit time, of the piezoelectric resonator). However, the relative large size of piezoelectric systems (approximately 1000 times the droplet size) requires correspondingly large separation between the nozzles of these systems. For example, Epson piezoelectric systems have about 20 nozzles per head, as compared to the 300 nozzles per head of thermal systems. Such prior systems thus sacrifice resolution for speed or speed for resolution.